Efficient light emission at a chosen wavelength, or combination of wavelengths, or a designed spectrum of wavelengths, is of great interest in many applications. In displays, it is desirable to be able to choose three distinct colours in the visible range. For lighting applications, tailoring of a single spectral profile is desired to achieve bright white light generation. For communications, it is desirable to generate and modulate light in the infrared range 1270 nm-1610 nm, as specified by the coarse wavelength-division multiplexing standard. In night vision, it is important to convert invisible (infrared light) signals into proportional, spatially-resolved signals which are visible. In all such applications, efficiency, brightness, and spectral control form key requirements.
Quantum dots, or colloidal quantum nanocrystals, are particles of semiconductor on the length scale 1 to 20 nm broadly, but preferably 2-10 nm, and can provide size-tunable luminescence spectra which the present inventors and others have shown to be customizable across the infrared and visible spectral ranges. When incorporated into semiconducting polymers, it is possible to inject charge into these matrix materials and ensure that either separate electrons and holes, or electron-hole pairs known as excitons, are transferred from the semiconducting polymer to the nanocrystals. This energy transfer process is a necessary step for subsequent net emission of light from the quantum dots. It is of critical importance in applications involving electroluminescence to ascertain and control the efficiency of energy transfer from the polymer to the nanocrystals.
Solution-processible devices based on colloidal quantum dots embedded in a semiconducting polymer matrix represent a promising basis for monolithic integration of optoelectronic functions on a variety of substrates including silicon, glass, III-V semiconductors, and flexible plastics. Reports of electroluminescence in the visible and infrared [Handbook of Organic-Inorganic Hybrid Materials and Nanocomposites (ed. H. S. Nalva), American Scientific Publishers, 2003], as well as photovoltaic [W. U. Huynh, X. Peng, A. P. Alivisatos, Adv. Mater. 11, 923 (1999)] and optical modulation [S. Coe, W.-K. Woo, M. Bawendi, V. Bulovic, Nature 420, 19 (2002)] phenomena in the visible, point to the possibility of combining a variety of useful optical and optoelectronic functions on a single platform.
Much work, including that on size-selective precipitation of nanocrystals to achieve the greatest possible monodispersity, has focused on narrowing emission, absorption, and modulation linewidths. Once such control over spectral properties has been achieved, it then becomes attractive to combine a number of different families of quantum dots in order to engineer a broader spectral shape: applications of such broadband or spectrally-engineered devices include multi-color light emitters for color displays; white light emitters for illumination; and, in the infrared, multi-wavelength emitters for coarse wavelength-division multiplexing and code-division multiple access [A. Stok, E. H. Sargent, IEEE Network 14, 42 (2000)}, useful in for example telecom integrated circuits (ICs).